Darth Nihilus

As support character, use a speed set and a potency set. The faster he plays, the faster he may drain force and land his damage over time.
Primary bonuses are mostly protection and potency. Survive to debuff !

As leader, you'll need to put an offense set with a potency set.
Due to his leader skill transforming protection in health, giving a lot of health steal but removing all chances of critical hits, you simply want as must offense as possible to have enough self-heal.

Primary bonuses are the same : protection as it will be converted to health (you gain a lot more protection than health with mods bonuses). You can add some offense (triangle) and potency (cross) depending on the opponents you face.

Sith Trooper

Due to his unique doubling his defense, you'd better stack some. Being able to taunt automatically you may mitigate a lot of damage this way.

For primary bonuses, a lot of protection is needed for the Sith Trooper, considering his special can restore some of his protection.
Finally with his offense down basic, some potency is also a good choice.

When playing with Palpatine as leader, forget all potency bonuses and go full protection !

Sith Assassin

the Sith Assassin is here to trigger a chain reaction, mostly when playing behind Darth Maul. Speed is the key to launch Dark Shroud as soon as possible (giving turn meter to Sith allies)
A potency set will help her debuff enemies.

For the primary bonuses, give her some protection to last into battle.